3.4.25 `out-pop' - close current output file

Usage: (out-pop [ disp ])
If there has been a push on the output, then close that file and go back to the previously open file. It is an error if there has not been a push. See section 3.7 Redirecting Output.

If there is no argument, no further action is taken. Otherwise, the argument should be #t and the contents of the file are returned by the function.

Arguments:
disp - Optional - return contents of the file